Merge multi-indexed with single-indexed data frames in pandas

I have two dataframes. df1 is multi-indexed:

                value
first second    
a     x         0.471780
      y         0.774908
      z         0.563634
b     x         -0.353756
      y         0.368062
      z         -1.721840

and df2:

      value
first   
a     10
b     20

How can I merge the two data frames with only one of the multi-indexes, in this case the 'first' index? The desired output would be:

                value1      value2
first second    
a     x         0.471780    10
      y         0.774908    10
      z         0.563634    10
b     x         -0.353756   20
      y         0.368062    20
      z         -1.721840   20